The present invention relates to a device for automatic, process-controlled feeding of electrolytic cells for producing aluminum, having a pressurized chamber for alumina and fluxing agents, feed pipe to the cells and bunker on each cell for storing alumina. The invention relates, too, to a method of operating the said device.
In the manufacture of aluminum from aluminum oxide the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The aluminum which separates out at the cathode collects under the fluoride melt on the carbon floor of the cell; the surface of this liquid aluminum acts as the cathode. Dipping into the melt from above are anodes which, in the conventional reduction process, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced at the carbon anodes, this oxygen combines with the carbon in the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C.
The concentration of aluminum oxide decreases in the course of the process. At an Al.sub.2 O.sub.3 concentration of 1-2 wt.% the so-called anode effect occurs suddenly producing an increase in voltage from 4--4.5 V to 30 V and more. It is then required that the crust must be broken open and the concentration of aluminum oxide increased by adding more alumina to the cell.
Under normal operating conditions the cell is fed with aluminum oxide regularly, even when no anode effect occurs. Also, whenever the anode effect occurs the crust must be broken open and the alumina concentration increased by the addition of more aluminum oxide, which is called servicing the cell.
For many years now servicing the cell includes breaking open the crust of solidified melt between the anodes and the side ledge of the cell, and then adding fresh aluminum oxide. This process which is still widely practiced today is finding increasing criticism because of the pollution of the air in the pot room and the air outside. In recent years therefore it has become increasingly necessary and obligatory to hood over or encapsulated the reduction cells and to treat the exhaust gases. It is however not possible to capture completely all the exhaust gases by hooding the cells if the cells are serviced in the classical manner between the anodes and the side ledge of the cells.
More recently therefore aluminum producers have been going over to servicing at the longitudinal axis of the cell. After breaking open the crust, the alumina is fed to the cell either locally and continuously according to the point feeder principle or discontinuously along the whole of the central axis of the cell. In both cases a storage bunker for alumina is provided above the cell. The same applies for the transverse cell feeding proposed recently by the applicant (U.S. Pat. No. 4,172,018).
The bunkers for storing alumina can be re-filled from a silo mounted on a pot room vehicle or cell manipulator.
In view of the large amounts of alumina consumed and the unavoidable dust put into the air by this method, attempts have been made to use pneumatic means of transport. Alumina transported in dilute flow conditions reaches transportation speeds of about 10 m/sec in such systems. With these high speeds, however, the material of the pipeline system is subject to extremely high rates of wear. In turn, the frequency changing of parts of the system results in technical and economic disadvantages. Furthermore, it has been found difficult to feed the necessary fluxing agents quickly and to the required place in a particular cell during operation of the cell.
It is therefore the principal object of the present invention to develop a device for the automatic, process controlled feeding of electrolytic cells for producing aluminum and a method of operating the said device which, while requiring a minimum of energy, the degree of wear on the raw materials employed is so low that the service life of the feed pipes equals or exceeds that of the cell. It is a further object that a fast, accurate feeding of fluxing agents to a particular cell is assured.